Ammonia has to be removed from gas streams that are vented into the air, because it causes environmental problems. Governmental regulations with respect to ammonia emissions will become stricter in future.
A process for the removal of ammonia from an ammonia containing gas stream is described, for instance, in U.S. Pat. No. 4,424,072.
In this patent specification is described that the ammonia is removed from the gas stream exiting from the top of a prilling tower in a urea plant by contacting the gas stream in a scrubber with a non-volatile, dilute acid solution whereby the NH3 is absorbed. According to U.S. Pat. No. 4,424,072, non-volatile acid solutions include inorganic acids such as phosphoric acid, sulphuric acid and nitric acid as well as organic acids such as citric acid, oxalic acid and comparable non-volatile organic acids. According to U.S. Pat. No. 4,424,072 the ammonia-free gas stream is vented to the air. It is mentioned that the obtained aqueous stream comprising the ammonium salt leaving the scrubbing section in the prilling tower can be recycled to the urea production process.
The drawback of using the process according to U.S. Pat. No. 4,424,072, wherein the aqueous stream comprising the ammonium salt is recycled to the urea production process, is that contaminations by ammonium salts may occur in the final product, in this case urea, which contaminations are undesirable. For instance, ammonium salts in urea will usually render this urea unsuitable for the preparation of melamine. The separate processing of these ammonium salts (e.g. as a by-product) costs money and energy or often poses an environmental problem.
It has been found that the aforementioned drawback can be eliminated with a process wherein the aqueous stream comprising the ammonium salt is treated with electrodialysis, whereby the acid is recovered and an aqueous stream comprising an ammonium hydroxide salt is formed.
By performing the electrodialysis on the aqueous stream comprising the ammonium salt the acid that has been used to remove the ammonia can be recovered and an aqueous stream comprising the ammonium salt is not recycled to the urea production causing contamination of the produced urea.